is then in the situation of the earth for the longest day in our northern hemisphere, the annual index pointing to the first point of Cancer, and the 21st of June; bring the meridian of London to coincide with the central solar ray, and move the hourcircle, NO, till the index points to XII; we then have the situation of London with respect to the longest day.

Now, on gently turning the handle of the machine, the point representing London will, by the rotation of the earth, be carried away towards the east, while the sun seems to move westward; and when London has arrived at the eastern part of the terminator, the index will point on the hour circle the time of sun-setting for that day; continue to turn on, and London will move in the shaded part of the earth, on the other side of the terminator; when the index is again at XII, it is midnight at London: by moving on, London will emerge from the western side of the terminator, and the index will point out the time of sun-rising, the sun at that instant appearing to rise above the horizon in the east to an inhabitant of London.

It will be evident by the instrument, while in this position, that the central solar ray, during the whole revolution of the earth on its axis, only points to the tropic of Cancer, and that the sun is vertical to no other parts of the earth, but those which are under this tropic.

By observing how the terminator cuts the several

parallels of the globe, we shall find that all those be'tween the northern and southern polar circles, except the equator, are divided unequally into diurnal and nocturnal arches, the former being greatest on the north side of the equator, and the latter on the south side of it.

In this position, the northern polar circle is wholly on that side the terminator which is nearest the sun, and therefore altogether in the enlightened hemisphere, and the inhabitants thereof enjoy a continual day. In the same manner, the inhabitants of the southern polar circle continue in the dark at this time, notwithstanding the diurnal revolution of the earth; it is the annual motion only which can relieve them from this situation of perpetual darkness, and bring to them the blessings of day, and the enjoyments of summer; while in this state, the inhabitants of north latitude are nearest to the central solar ray, and consequently to the sun's perpendicular beams, and of course a greater number of his rays will fall upon any given place, than at any other time; the sun's rays do now also pass through a less quantity of the atmosphere, which, together with the length of the day and the shortness of the night, are the reasons of the increase of heat in summer, together with all its other delightful effects.

While the earth continues to turn round on its own axis once a day, it is continually advancing from west to east, according to the order of the signs, as is seen by the progress of the annual

index E, which points successively to all the signs and degrees of the ecliptic: the sun in the mean time seems to describe the ecliptic also, going from west to east, at the distance of six signs from the earth; that is, when the earth really sets out from the first point of Capricorn, the sun seems to set out from the first point of Cancer, as is plain from the index :

But as, during the annual revolution of the earth, the axis always remains parallel to itself, the situation of this axis, with respect to the sun, must be continually changing.

As the earth moves on in the ecliptic, the northern polar circle gets gradually under the terminator, so that when the earth has arrived at the first point of Aries, and the annual index is at the first point of Libra on the 22d of September, this circle is divided into two equal parts by the terminator, as is also every other parallel circle, and consequently the diurnal and nocturnal arches are equal: this is called the time of equinox, the days and nights are then equal all over the earth, being each of them 12 hours long, as will be seen by the horary index L. The central solar ray G, having successively pointed to all the parallels that may be supposed to be between the equator and the tropic of Cancer, is at this period perpendicular to the inhabitants that live at the equator.

By continuing to turn the handle, the earth advances in the ecliptic, and the terminator shews how the days are continually decreasing, and the diurnal

arches shortening, till by degrees the whole space contained by the northern polar circle is on that side of the terminator which is opposite to the sun, which happens when the earth is got to the first point of Cancer, and the annual index is at the first point of Capricorn, on the 21st of December. In this state of the globe, the northern polar circle, and all, the country within that space, have no day at all; whilst the inhabitants that live within the southern polar circle, being on that side the terminator which is next the sun, enjoy perpetual day. By this, and the former situation of the earth, the pupil will observe that there are nations to whom a great portion of the year is darkness, who are condemned to pass weeks and months without the benign influence of the solar rays. The central solar ray is now perpendicular to the tropic of Capricorn; the length of the days is now inversely what it was when the sun entered Cancer, the days being now at their shortest and nights longest in the northern hemisphere; the length of each is pointed out by the horary index.

The earth being again carried on till it enters Libra, and the sun Aries, we shall again have all the phenomena of the equinoctial seasons. The terminator will divide all the parallels into twó equal parts; the poles will again be in the plane of the terminator, and consequently, as the globe revolves, every place from pole to pole will describe an equal arch in the enlightened and obscure he

mispheres, entering into and going out of each exactly at six o'clock, as shewn by the hour-index.

As the earth advances, more of the northern polar circle comes into the illuminated hemisphere, and consequently the days increase with us, while those on the other side of the equator decrease, till the earth arrives at the first point of Capricorn, the place from which we first began to make our observations.

To explain the phenomena that take place in a parallel, direct, and right sphere.

Take off the globe and its terminator, and put on in its place the globe which accompanies the instrument, and which is furnished with a meridian, horizon, and quadrant of altitude; the edge of the horizon is graduated from east and west to the north and south points, and within these divisions are the points of the compass to the under side of this horizon; but at 18 degrees from it another circle is affixed, to represent the twilight circle; the meridian is graduated like the meridian of a globe; the quadrant of altitude is divided into degrees, beginning at the zenith, and finishing at the horizon.*

* We now more properly apply the moveable horizon, &c. to all the 3-inch globes of these sort of orreries, so that the trouble of changing the globe is done away. EDIT.